1. PLATE TECTONICS,
EARTHQUAKES, AND
VOLCANISM

4. PLATE TECTONIC
THEORY

6. • Based on a model of earth in which rigid
lithosphere, consisting of oceanic crust as well as
continental crust as well as about 100km of the
underlying upper mantle, is made up of a number of
pieces known as plates
• Lithospheric plates move over the hotter and weaker
semi-plastic asthenosphere below. As plates move, they
separate, mostly at mid-oceanic ridges, while colliding
in other areas.
• When plates collide, one plate generally dives beneath
the other, although in some cases the plates simply
slide past one another.

8. CONTINENTAL
DRIFT THEORY

9. • In 1915, Alfred Wegener published “The Origin of Continents and
Ocean”
• Wegener suggested that a supercontinent called Pangaea
(meaning all land), once existed
• About 200 million years ago, this supercontinent began breaking
into smaller continents, which then “drifted” to their present
positions
• Wegener and others collected substantial evidence to support
these claims. The evidences includes:
• The fit of South America and Africa
• The geographic distribution of fossils
• Rock structures
• Ancient climates

10. Evidences of CDT

11. • Wegener first suspected that the continents night have
been joined when he noticed the remarkable similarity
between the coastlines on opposite sides of the South
Atlantic

12. • He learned that
identical fossils or
organisms were
known from rocks in
both South America
and Africa
• He believed that
there once existed a
transoceanic bridges

13. • If the continents were once together, the rocks
found in a particular region on one continent should
closely match in age and type those found in adjacent
positions on the adjoining continent

14. • He was keenly interested in
paleoclimatic (paleo – ancient)
data to support continental
drift
• He learned of ancient glacial
deposits that indicated that
near the end of Paleozoic Era
(about 300 million years ago),
ice sheets covered extensive
areas of the Southern
Hemisphere and India
• Layers of glacially transported
sediments of the same age
were found in Southern Africa
and South America, as well as
India and Australia

15. As late as 1953 – just five years before Carey introduced the theory
of plate tectonics – the theory of continental drift was rejected by the
physicist Scheiddiger on the following grounds.
• First, it had been shown that floating masses on a rotating geoid
would collect at the equator, and stay there. This would explain
one, but only one, mountain building episode between any pair
of continents; it failed to account for earlier orogenic episodes.
• Second, masses floating freely in a fluid substratum, like
icebergs in the ocean, should be in isostatic equilibrium (where
the forces of gravity and buoyancy are in balance). Gravitational
measurements were showing that many areas are not in
isostatic equilibrium.
• Third, there was the problem of why some parts of the Earth's
surface (crust) should have solidified while other parts were still
fluid. Various attempts to explain this foundered on other
difficulties.

16. Someday …
… will the continents come back
together and form a single
landmass ?

17. SEAFLOOR
SPREADING THEORY

19. • Seafloor spreading is a process that
occurs at mid-ocean ridges, where
new oceanic crust is formed
through volcanic activity and then
gradually moves away from the
ridge.
• Seafloor spreading helps explain
continental drift in the theory of
plate tectonics. When oceanic
plates diverge, tensional stress
causes fractures to occur in the
lithosphere. Basaltic magma rises
up the fractures and cools on the
ocean floor to form new sea floor.
• Older rocks will be found
further away from the
spreading zone while younger
rocks will be found nearer to
the spreading zone.

20. Age of oceanic crust; youngest (red) is along spreading
centers.

21. PLATE TECTONICS
Plate and
Plate Boundaries

22. • Lithospheric plates move a coherent units relative to all
other plates.
• Earth’s surface consist of great areas, or plates, that are
geologically quiet, but are separated by narrow zones
of seismic and volcanic activity where the plates
separate, collide, or slide past one another.
• Plates are bounded by three distinct types of
boundaries, which are differentiated by the type of
movement they exhibit.

23. • Constructive Margins
• Where two plates move apart, resulting in up-welling of
material from the mantle to create new seafloor
• Upwelling
• Rifting

24. • Destructive Margins
• Where two plates move together, resulting in oceanic
lithosphere descending beneath an overriding
plate, eventually to be reabsorbed to the mantle, or
possibly in the collision of two continental blocks to
create a mountain system
• Oceanic-Oceanic
• Oceanic-Continental
• Continental-Continental

25. • Conservative Margins
• Where plates grind past
each other without the
production or
destruction of
lithosphere.

28. Forces that Drive
Plate Motion

29. • The basic process responsible for plate motion

30. • The subduction of cold, dense slabs of oceanic
lithosphere is the main driving force of the
plate motion
• As these slabs sinks into the asthenosphere, it
“pull” the trailing plate along
• Results because old slabs of oceanic lithosphere
are more dense than the underlying
asthenosphere and hence “sink like a rock”

31. • Results from the elevated position of the
oceanic ridge, which causes slabs of lithosphere
to slide down the flanks of ridge
• Ridge push appears to contribute far less to
plate motions than does slab pull

32. • Arises from the drag of a subducting slab on the
adjacent mantle
• The result is an induced mantle circulation that
pulls both the subducting and overriding plates
toward the trench

34. What drives plate
motion ?

35. 1. Convective flow in the rocky 2900km thick mantle is
the underlying driving force for plate movement
2. Mantle convection and plate tectonics are part of the
same system. Subducting oceanic plates drive the cold
downward-moving portion of convective flow where
as shallow upwelling of hot rock along the oceanic
ridge and buoyant mantle plumes are the upward-
flowing arm of the convective mechanism.
3. The slow movements of Earth’s plates and mantle are
ultimately driven by the unequal distribution of heat
within Earth’s interior.

36. Measuring Plate
Motion

37. • Very Long Baseline Interferometry
• Utilizes large radio telescopes to record signals
from very distant quasars (quasi- stellar
objects), quasars acts as stationary reference
points.
• Useful in establishing large-scale plate motions

38. • Global Positioning System
• Uses 21 satellites to accurately locate any
individual who is equipped with a handheld
receiver
• Useful in establishing small-scale crustal
movements as those that occur along local
faults in regions known to be tectonically active

39. The Theory of Plate
Tectonic is supported by:

40. 1. Paleomagnetism, the direction and intensity of
Earth’s magnetism in the geologic past.
2. The global distribution of earthquakes and
their close association with plate boundaries.
3. The ages of sediments from the floors of deep
ocean basins.
4. The existence of island groups that formed
over hotspots and that provide a frames of
reference for tracing the direction of plate
motion.